1. Field of the Invention
This invention relates to a method and apparatus for making a self-coiling sheet.
2. Description of the Prior Art
The prior art is replete with methods and apparatus for making a self-coiling sheet from a flexible sheet material. For example, U.S. Pat. No. 2,053,811 describes a method and apparatus for making such a sheet by impregnating a sheet of paper, jute, etc., with a binder such as a phenolformaldehyde condensation product. The binder is then curved by the application of heat and pressure to the sheet material. While the material is still hot, it is rolled into a tube of the desired dimensions and is held in such shape until it cools. The resulting product may then be used as a protective shield for electrical conductors which, when applied to the conductor, automatically contracts around the conductor and firmly engages it. Also, see U.S. Pat. No. 2,826,523 wherein a self-coiling protective covering is comprised of a laminated sheet having a plurality of layers, at least one of which is resilient and prestressed before it is bonded to adjacent layers whereby the sheet will roll upon itself tightly and automatically. Still another example is set forth in U.S. Pat. No. 3,426,115 wherein a sheet of crystalline polyethylene terephthalate is rolled into a coil, heated while held in the coiled condition, and then released when the sheet material has cooled. The resulting product may be used as a self-retracting shelf for supporting packaged goods.
Lately, self-coiling sheets have found utility in cameras of the self-developing type wherein they are used to temporarily shield a film unit from the ambient light as it exits from the camera. Examples of the latter may be found in U.S. Pat. Nos. 3,940,774 and 4,072,968. Basically, these self-coiling sheets are made from sheets of Mylar (crystalline polyethylene terephthalate) or Tedlar (polyvinyl fluoride) having a suitable opacifier, e.g., carbon black, therein. The sheets, which are preferably 0.0038 centimeters thick, are rolled into a coil, placed in tubes, and heated to a predetermined temperature for a predetermined period of time. The sheets are then allowed to cool before they are removed from their respective tubes.
From the foregoing, it can be seen that there is room for improvement in the methods and apparatus for making a self-coiling sheet. For example, most, if not all, of the methods described above require a period of time during which the sheet must be maintained in a certain condition until its temperature has declined to a predetermined level. Obviously, this increases the time that it takes to make a self-coiling sheet. Accordingly, it can be seen that there is room for improvement in such methods and apparatus.